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A marine is spawned each time you make a move, i.e. remove three jewels. The only way you could make more marines is to make more moves.

As in Starcraft itself, StarJeweled makes you pay attention to different things - the jewel game and the tug of war. It usually helps a lot if you are able to focus on jewels. One way to do this is to just spam one unit in tug of war and never think about it again. Another way is to ask another person to play tug of war while you concentrate on jewels.

As for the game itself, try to make moves in different parts of the field, or go top to bottom, to ensure there are no falling jewels in the area you are looking at. If you can't see a move for a second or two, just shuffle the board, there's no time to look closer.